1. Field of the Invention
This invention relates to a method for manufacturing a photosensitive member for electrophotography composed of amorphous silicon, amorphous silicon carbide or the like (hereinafter referred to as "a-Si") with improved reproducibility and stability.
2. Description of the Related Art
In the field of image forming, there is a need for photoconductive materials forming photosensitive members for electrophotography to have certain characteristics: a high sensitivity, a high S/N ratio photocurrent (Ip)/dark current (Id)!, an absorption spectrum matching with spectrum characteristics of irradiated electric waves, a fast photo-response, a desired dark resistance value, and causing no harm to the human body during use. In the case of an electrophotographic photosensitive member incorporated in electrophotographic apparatuses used for office work or the like, the quality of being harmless during use mentioned above is particularly important. It is also important that the electrophotographic photosensitive member should have little impact upon the environment after the completion of the useful life of the apparatus.
From this viewpoint, attention has been directed again to amorphous silicon as a photoconductive material. For example, in German Patent Laid-Open Publication Nos. 2746967 and 2855718, applications of amorphous silicon as an electrophotographic photosensitive material are described.
In German Patent Laid-Open Publication No. 3046509 there is proposed an electrophotographic photosensitive member formed of an electroconductive support and an a-Si photoconductive layer having halogen atoms as a constituent. According to the art disclosed in this publication, electrical and optical characteristics suitable for a photosensitive layer of an electrophotographic photosensitive member can be achieved by adding 1 to 40 atomic percent of halogen atoms to a-Si.
On the other hand, amorphous silicon carbide (hereinafter referred to as "a-SiC") is known as a material having high heat resistance and high surface hardness, having a high dark resistivity in comparison with a-Si and capable of exhibiting an optical band gap through the range of 1.6 to 2.8 eV by selecting the carbon content. In U.S. Pat. No. 4,471,042 there is proposed an electrophotographic photosensitive member having a photoconductive layer formed of such a-SiC. In the art disclosed in this publication, a photoconductive layer of an electrophotographic photosensitive member is formed by using a-Si containing 0.1 to 30 atomic percent of carbon as a chemical modifier to achieve improved electrophotographic characteristics, i.e., high dark resistance and high photo-sensitivity.
Further, in Japanese Patent Publication No. 63-35026 there is proposed an electrophotographic photosensitive member having, on an electroconductive support, an intermediate layer of a-Si having carbon atoms and hydrogen atoms and/or fluorine atoms as constituents (hereinafter referred to as "a-SiC (H, F)", and an a-Si photoconductive layer. The a-SiC (H, F) intermediate layer containing at least hydrogen atoms and/or fluorine atoms is provided to reduce cracks and separation of the a-Si photoconductive layer without impairing the desired photoconductive characteristics.
Plasma chemical vapor deposition (CVD) processes and sputtering processes are known as processes for forming such a-Si photosensitive members. Among plasma CVD processes used for this purpose, RF plasma CVD (RF-PCVD) processes are known as an ordinary forming process. An apparatus and a method for forming a deposited film by an RF-PCVD process will be described below.
FIG. 1 is a schematic diagram showing an example of an apparatus for manufacturing an electrophotographic photosensitive member supported on RF-PCVD. This apparatus is mainly constituted of a deposition unit 2000, a raw material gas supply unit 2200, and an evacuation unit (not shown) for reducing the internal pressure of a reaction chamber 2111. In the reaction chamber 2111 of the deposition unit 2000, a heater 2113 for heating a support 2112 and pipes 2114 for introducing raw material gases having raw material gas introducing holes are provided. A high frequency matching box 2115 is connected to the reaction chamber 2111. The support 2112 on which a layer or layers of a-Si photosensitive member are formed has, for example, a cylindrical shape such as that shown in FIG. 1, and is placed in a desired position in the reaction chamber 2111.
The raw material gas supply unit 2200 has bombs 2221 to 2226 containing raw material gases necessary for forming the desired layers, e.g., SiH.sub.4, H.sub.2, CH.sub.4, NO, NH.sub.3, and SiF.sub.4, valves (2231 to 2236, 2241 to 2246, 2251 to 2256), and mass flow controllers 2211 to 2216. The raw material gas bombs are connected to gas introducing pipes 2114 in the reaction chamber 2111 through a valve 2260.
A deposited film can be formed by using this apparatus, for example, as described below. A cylindrical support 2112 is first placed in the reaction chamber 2111, and the interior of the reaction chamber 2111 is evacuated by the evacuation unit, not shown (e.g., a vacuum pump). Then, the temperature of the cylindrical support 2112 is controlled with the supporting member heater 2113 to be maintained at a predetermined temperature of 20.degree. to 500.degree. C.
To introduce raw material gases for forming a deposited film into the reaction chamber 2111, the closed state of each of the valves 2231 to 2236 of the gas bombs and a leak valve 2117 of the reaction chamber is confirmed, the opened state of each of the outflow valves 2251 to 2256 and the auxiliary valve 2260 is also confirmed, and a main valve 2118 is thereafter opened to evacuate the reaction chamber 2111 and a gas piping 2116.
Next, when a vacuum meter 2119 reads about 5.times.10.sup.-5 Torr, the auxiliary valve 2260 and the outflow valves 2251 to 2256 are closed.
Thereafter, the gases are introduced from the gas bombs 2221 to 2226 by opening the valves 2231 to 2226, and the pressure of each gas is regulated at 2 kg/cm.sup.2 with pressure regulators 2261 to 2266. Then, the inflow valves 2241 to 2246 are gradually opened to introduce the gases into the mass flow controllers 2211 to 2216.
After a film forming preparation step has been completed in the above-described manner, and when the temperature of the cylindrical support 2112 is maintained at the desired temperature, the needed outflow valves 2251 to 2256 and the auxiliary valve 2260 are gradually opened to introduce desired gases from the gas bombs 2221 to 2226 into the reaction chamber 2111 through the gas introducing pipes 2114.
Next, the flow rates of the raw material gases are controlled so as to be set to predetermined values by the mass flow controllers 2211 to 2216. Simultaneously, the opening of the main valve 2118 is adjusted while reading the vacuum meter 2119 so that the pressure in the reaction chamber 2111 is set to a desired pressure not higher than 1 Torr. When the internal pressure of the reaction chamber 2111 is stabilized, power from an RF power source (not shown) is set to a predetermined level, and the RF power is introduced into the reaction chamber 2111 through the high-frequency matching box 2115 to cause RF glow discharge. By the energy of this discharge, the raw material gases introduced into the reaction chamber 2111 are decomposed and a deposited film having predetermined silicon as a main component is formed on the cylindrical support 2112. When the thickness of the formed film becomes equal to a desired thickness, the outflow valves are closed to stop the gas flow into the reaction chamber 2111, thereby terminating the formation of the deposited film.
The same operations are repeated a certain number of times to form a desired multilayer photosensitive member.
Needless to say, when each layer is formed, the outflow valves other than those for introducing necessary gases are closed. Also, the operation of closing the outflow valves 2251 to 2256, opening the auxiliary valve 2260 and fully opening the main valve 2118 to evacuate the system to high vacuum is performed to remove remaining gases in the reaction chamber 2111 and in the piping between the outflow valves 2251 to 2256 and the reaction chamber 2111, if necessary.
To improve the uniformity of film formation, the cylindrical support 2112 may be rotated at a desired speed by a drive unit (not shown) during film formation.
Needless to say, the kinds of gas and the valve operation described above may be changed according to conditions of formation of each layer.
A heating member adapted to use in vacuum may be used as a means for heating the support. Examples of the heating member are a sheath type wound heater, a plate-like heater, an electrical resistance heating member, such as a plate heater or a ceramic heater, a heat radiating lamp heating member, such as a halogen lamp or an infrared lamp, and a heating member having heat exchange means using a liquid or a gas as a thermal medium. As a surface material of the heating means, a metal, such as stainless steel, nickel, aluminum or copper, a ceramic, a heat-resistant macromolecular resin or the like may be used. Other heating means may also be used. For example, a special heating chamber other than the reaction chamber may be provided to heat the support, and the heated support may be transported into the reaction chamber while being maintained in a vacuum.
Next, an apparatus and a method for forming a deposited film supported on another kind of plasma CVD process known as a microwave plasma CVD process will be described. FIG. 2 is a schematic diagram of the construction of an example of a reactor for forming a deposited film for an electrophotographic photosensitive member by a microwave plasma CVD (hereinafter referred to as ".mu.W-PCVD"), and FIG. 3 is a schematic cross-sectional view of the reactor.
An apparatus for manufacturing an electrophotographic photosensitive member by .mu.W-PCVD, constructed as described below, can be formed by connecting a deposition unit 3100 shown in FIG. 2, which is used in place of the deposition unit 2000 for RF-PCVD in the manufacturing apparatus shown in FIG. 1, to the raw material gas supply unit 2200.
This apparatus is constituted of a reaction chamber 3111 having a vacuum airtight structure and capable of being evacuated, the raw material gas supply unit 2200, and an evacuation unit (not shown) for decompressing the interior of the reaction chamber. In the reaction chamber 3111 are provided microwave introducing windows 3112 formed of a material through which microwave electric power can be efficiently transmitted to the interior of the reaction chamber and which can maintain a vacuum airtight condition (e.g., quartz glass, alumina ceramic or the like), microwave waveguides 3113 connected to a microwave power source (not shown) through a stab tuner (not shown) and an isolator (not shown), support heaters 3116, a raw material gas introducing pipe 3117, and an electrode 3118 for applying an external electrical bias for controlling a plasma potential. The interior of the reaction chamber 3111 communicates with a diffusion pump (not shown) through an evacuation pipe 3121. The raw material gas supply unit 2200 has bombs 2221 to 2226 containing necessary raw material gases, e.g., SiH.sub.4, H.sub.2, CH.sub.4, NO, NH.sub.3, and SiF.sub.4, as in the case of RF-PCVD, valves (2231 to 2236, 2241 to 2246, 2251 to 2256), and mass flow controllers 2211 to 2216. The raw material gas bombs are connected to the gas introducing pipe 3117 in the reaction chamber 2111 through a valve 2260. A space 3130 surrounded by cylindrical supports 3115, which are placed in the reaction chamber 3111 and on which a layer or layers of an amorphous silicon photosensitive member is formed, is formed as a discharge space.
A deposited film can be formed by .mu.W-PCVD using this apparatus as described below. Cylindrical supports 3115 are first placed in the reaction chamber 3111 and are rotated by driving devices 3120, and the interior of the reaction chamber 3111 is evacuated by the unillustrated evacuation unit (e.g., a vacuum pump) through the evacuation pipe 3121. The pressure in the reaction chamber 3111 is controlled so as to be maintained at 1.times.10.sup.-6 Torr or lower. Then, the temperature of the cylindrical supports 3115 is increased and maintained at a predetermined temperature of 20.degree. to 500.degree. C. by the support heaters 3116.
To introduce raw material gases for forming a deposited film into the reaction chamber 3111, the closed state of each of the valves 2231 to 2236 of the gas bombs and a leak valve of the reaction chamber is confirmed, the opened state of each of the outflow valves 2251 to 2256 and the auxiliary valve 2260 is also confirmed, and a main valve (not shown) of the reaction chamber 3111 is thereafter opened to evacuate the reaction chamber 3111 and gas pipe system of the same.
Next, when a vacuum meter (not shown) reads about 5.times.10.sup.-5 Torr, the auxiliary valve 2260 and the outflow valves 2251 to 2256 are closed.
Thereafter, the gases are introduced from the gas bombs 2221 to 2226 by opening the valves 2231 to 2226, and the pressure of each gas is regulated at 2 kg/cm.sup.2 with pressure regulators 2261 to 2266. Then, the inflow valves 2241 to 2246 are gradually opened to introduce the gases into the mass flow controllers 2211 to 2216.
After a film forming preparation step has been completed in the above-described manner, and when the temperature of the cylindrical supports 3115 is maintained at the desired temperature, necessary ones of the outflow valves 2251 to 2256 and the auxiliary valve 2260 are gradually opened to introduce desired ones of the gases from the gas bombs 2221 to 2226 into the reaction chamber 3111 through the gas introducing pipe 3117.
Next, the flow rates of the raw material gases are controlled so as to be set to predetermined values by the mass flow controllers 2211 to 2216. Simultaneously, the opening of the main valve is adjusted while reading the vacuum meter so that the pressure in the reaction chamber 3111 is set to a predetermined pressure not higher than 1 Torr. When the internal pressure of the reaction chamber 3111 is stabilized, microwaves having a frequency of 500 MHz or higher, more preferably 2.45 GHz, are generated by the microwave power source, the power from the microwave power source is set to a desired level, and microwave energy is introduced into the discharge space 3130 through the wave guides 3113 and microwave introducing windows 3112 to cause microwave glow discharge. Simultaneously, an electric bias, e.g., a direct current, is applied to the electrode 3118 from a power source 3119. As a result, in the discharge space 3130 surrounded by the supports 3115, the introduced raw material gases are dissociated by being excited with the microwave energy to form the desired deposited film on the cylindrical supports 3115. At this time, to improve the uniformity of film formation, the cylindrical supports 3115 are rotated at a desired speed by support rotating motors 3120.
When the thickness of the formed film becomes equal to a desired thickness, the supply of microwaves is stopped and the supply of the gases to the reaction chamber is stopped by closing the outflow valves, thereby terminating the formation of the deposited film.
The same operations are repeated a certain number of times to form a desired multilayer photosensitive member.
Needless to say, when each layer is formed, the outflow valves other than those for introducing necessary gases are closed, as in the case of RF-PCVD. Also, the operation of closing the outflow valves 2251 to 2256, opening the auxiliary valve 2260 and fully opening the main valve to evacuate the system to high vacuum is performed to remove remaining gases in the reaction chamber 3111 and in the piping between the outflow valves 2251 to 2256 and the reaction chamber 3111, if necessary.
Needless to say, the kinds of gas and the valve operation described above may be changed according to conditions of formation of each layer.
A heating member adapted to use in vacuum may be used as a means for heating the support, as in the case of the above-described RF-PCVD process. Alternatively, a special heating chamber other than the reaction chamber may be provided to heat the support, and the heated support may be transported into the reaction chamber while being maintained in a vacuum.
In the .mu.W-PCVD process, the pressure in the discharge space is set, preferably, in the range of 1.times.10.sup.-3 to 1.times.10.sup.-1 Torr, more preferably, in the range of 3.times.10.sup.-3 to 5.times.10.sup.-2 Torr, and most preferably, in the range of 5.times.10.sup.-3 to 3.times.10.sup.-2 Torr.
The pressure outside the discharge space may be set to any pressure lower than that in the discharge space. However, the effect of improving the properties of the deposited film is particularly high, if the pressure in the discharge space is three times the pressure outside the discharge space or higher when the pressure in the discharge space is 1.times.10.sup.-1 Torr or lower, more particularly when it is 5.times.10.sup.-2 Torr or lower.
Microwaves are guided to the reactor, for example, by a method of using a waveguide, and are introduced into the reactor, for example, by a method of introduction through one or a plurality of dielectric windows. Such a microwave introducing window is ordinarily formed of a material of a small microwave loss, such as alumina (Al.sub.2 O.sub.3), aluminum nitride (AlN) , boron nitride (BN), silicon nitride (SiN), silicon carbide (SiC), silicon oxide (SiO.sub.2), berylium oxide (BeO), Teflon or polystyrene.
The waveform and frequency of the voltage applied to the electrode are not particularly limited and the size and shape of the electrode may be selected freely as long as discharge is not disturbed. For practical use, it is preferable to form the electrode into the shape of a cylinder having a diameter in the range of 0.1 to 5 cm. Also, the length of the electrode may be set arbitrarily as long as an electric field is thereby applied uniformly to the support.
The electrode may be formed of any material as long as it has an electroconductive surface. Ordinarily, the electrode is, for example, a member formed of a metal, such as stainless steel, Al, Cr, Mo, Au, In, Nb, Te, V, Ti, Pt, Pd or Fe, an alloy of these metals, or a glass, ceramic or plastic member having a processed electroconductive surface.
FIG. 4 is a schematic cross-sectional view of an example of a-Si photosensitive members formed in the above-described manner. The illustrated photosensitive member has an electroconductive support 501 formed of Al or the like, a charge injection obstruction layer 502 for obstructing injection of charge from the electroconductive support 501, which is formed if necessary, a layer 503 formed of an amorphous material containing at least silicon atoms and having a photoconductive property, and a surface layer 504 provided if necessary by being formed of a material containing silicon atoms and carbon atoms and, if necessary, hydrogen atoms and/or halogen atoms. The surface layer 504 has a function of retaining electric charge and/or a function of improving characteristics relating to external factors, such as wear resistance and moisture resistance.
As described above, good electrophotographic photosensitive members have been manufactured by the electrophotographic photosensitive member manufacturing methods using RF-PCVD and .mu.W-PCVD processes. However, there is a need to provide an electrophotographic photosensitive member having further improved performance in order to meet various demands with respect to recent electrophotographic apparatuses, for example, for a further reduction in the size of the apparatus, an increase in the operating speed of the apparatus and an improvement in image quality.
More specifically, when the size of an electrophotographic apparatus is reduced, a main charging device is also reduced in size with a reduction in the size of the apparatus. As a result, the corona current supplied to a photosensitive member is reduced. Therefore, it is necessary to achieve a further improvement in charging performance of the photosensitive member in order to obtain a required dark potential.
With respect to realization of high speed image formation, an improvement in chargeability of a photosensitive member is inevitably required with a reduction in charging time. Further, the photo-response must also be improved since the time period taken to transport a photosensitive member after the formation of a latent image by irradiation of image exposure light to a development device for developing the latent image is reduced.
With respect to improvements in image qualities, it is required that image defects such as spot defects appearing as black or white dots in an image should be reduced and that occurrence of ghosts or the like, i.e., a phenomenon of density unevenness caused in an image by a preceding-time latent image remaining as a photo-memory during image formation repeated a number of times, should be further reduced.